My dreams have come true.
For the first time that I know of since World War II, there is serious talk of a production airplane that's powered by a diesel engine. Diamond Star, who built the Katana light airplane, has joined forces with Thielert Aircraft Engines, who took a 1.7 litre Daimler-Benz turbo diesel and retrofitted it for aviation use, and they plan to start selling it in Europe pretty quick, and it sounds like they may bring it over to America next year. They also have retrofit kits available for both Piper Cherokees and Cessna 172's.
At max economical cruise speed (110 knots), the engine burns 3 gallons an hour, which translates roughly into 42 miles to a gallon. In contrast, a Cessna typically gets about 20mpg. Also, this engine'll run on either auto diesel fuel or Jet-A fuel. At WOT, the engine burns 10.7 gallons an hour, which makes for a cruising speed of 181 knots and 19mpg.
Here's the link to the story:
http://machinedesign.com/ASP/viewSelectedArticle.asp?strArticleId=55900&strSite=MDSite&Screen=CURRENTISSUE